1. Field of the Invention
The present invention relates to technology for separating particles with light.
2. Related Background Art
Various methods have been known for separating particles such as living-organism-related particles, i.e., cells, microorganisms, liposomes, etc. and synthetic particles, i.e., latex particles, gel particles, industrial particles, etc.
As an example, there is a report on separating particles using so-called laser trapping, in which particles are confined in a space by collecting and condensing laser beams [H. Misawa, et al. Chem. Lett., 469(1991)].
The laser trapping is technology for handling the particles, utilizing a dynamic action of light on the particles. When a light beam having an intensity gradient, such as a laser beam, is condensed and irradiated onto particles, two types of forces act on the particles, i.e., a light pressure (a radiation pressure) acting in the irradiation direction of light beam and a force to confine the particles in the optical axis (i.e., a light gradient force). Either one of the light pressure and the light gradient force depends on the light intensity, an intensity distribution in the direction of the optical axis, which is a condensing degree by a lens and the like, and an intensity distribution in the direction perpendicular to the optical axis. The light pressure and the light gradient force further depend on the refractive index, the absorption index (reflectance) and the size of particles. Particles can be trapped at an irradiated position by the action of the gradient force among the two types of forces.
The separation of particles using the above-described laser trapping is conducted as follows. Prepared is a group of particles consisting of a mixture of two types of polystyrene latex particles which are different in size from each other. Laser beams are collected and condensed in a pattern of multiple rings by optical interference so as to impinge upon the particles. If the diameters of the rings are changed while keeping particles light-trapped on associated rings, smaller particles as weakly trapped spring out and are removed from the rings, whereby only larger particles are continuously trapped on the rings. As a result, the larger particles can be selectively separated.